The present invention relates in general to manufacturing processes that require lithography and, in particular, to methods of designing photomasks and optimizing lithographic and etch processes used in microelectronics manufacturing.
There is a continuing effort to reduce the dimensions of integrated circuit (IC) devices, and this has required greater precision in the tools used to manufacture IC devices. IC devices are designed using computer-aided design (CAD) or technology-computer-aided design (TCAD) layout tools which allow designers and manufacturers to plan the layout of circuits on a semiconductor wafer. The finished designs must be transferred to the wafer in a manner that allows device features to be produced by various processes of etching, depositing, implanting, and the like. This is done by applying a photoresist (also known as resist) layer to the surface of the wafer and then exposing the photoresist to radiation transmitted through a mask or reticle having patterns of transparent and opaque areas according to the feature or features to be formed on the wafer. The exposed photoresist is developed to provide openings in the photoresist layer through which the surface of the wafer is exposed for the process desired. This process of transferring the pattern to the wafer is generally referred to as photolithography. The finished product typically includes a number of patterned layers formed on the wafer, in which the patterns in different layers (or levels) are aligned to allow the formation of IC devices and circuit interconnection. Each patterned level or layer is typically formed using a separate mask or reticle layout pattern designed to form the desired pattern for that patterned level or layer.
The overall resolution of a photolithographic process refers to the minimum feature size (i.e., typically a critical dimension, or CD) that can be adequately printed, or “resolved,” within specifications. This overall resolution limit depends on the resolution of the optical lithographic system, the properties of the resist as well as the subsequent etch processes. The resolution of the lithography (optical) system, that is, the ability to form a resolvable image pattern on the wafer, is critical to the overall process, and can be improved by resolution enhancement techniques (RETs), including modifications of the mask or reticle, as discussed in more detail below.
The resolution R of an optical lithography system is defined to be the smallest feature size of a grating that is resolvable as a function of illumination wavelength λ and a numerical aperture NA, as expressed as R=k1λ/NA, where k1 is a process constant. For conventional optical lithography, the ultimate resolution limit is reached at k1=0.5, the state at which only one set of diffracted orders can pass through the imaging optical system. Even as exposure wavelengths continue to decrease, and numerical apertures continue to increase, conventional optical lithography is still challenged by resolution below k1=0.5. Approaching k1=0.5 imposes formidable problems due to image quality degradation associated with the loss of increasing numbers of diffracted orders.
At low k1 imaging, significant modifications to mask designs are required to print features in the desired fashion on the wafer. Due to the extreme sensitivity of many of these features to errors on the mask, in the stepper lens or in the lithography process (e.g., focus and dose), it is critical that these mask design modifications, or resolution enhancement techniques (RETs), be done properly. Resolution enhancement techniques such as optical proximity correction (OPC), subresolution assist feature enhancement (SRAF) lithography and phase-shifted-mask-enhanced (PSM) lithography have become increasingly important as resolution has increased beyond the quarter-micron level. In addition, RETs have been combined with the use of off-axis illumination (OAI) and advanced resist processing to bring the k1 value closer to 0.25.
Off-axis illumination (OAI) provides resolution enhancement by modifying the illumination direction incident on the mask so as to eliminate or reduce on-axis illumination. For on-axis (i.e., propagation along the optical axis) light incident on a grating having pitch P, the mth diffracted order will propagate at an angle □m=sin−1(mλ/P). However, only non-zero diffracted orders contain information about the grating, so at least one non-zero order must be collected in order to form an image. In other words, the projection lens must be large enough to collect at least the first order diffracted beams as well as the zero order beam. For the case of on-axis illumination, the first order diffracted beams m=−1 and m=+1 will propagate at angles □m=sin−1(λ/P) relative to the optical axis, and thus the smallest pitch will be limited by the ability of the optical system to collect at least 3 beams (i.e., m=−1, 0, and +1), that is, a projection lens capable of collecting orders subtending an angle 2□1. For a grating having equal lines and spaces having pitch P and line widths (CD) equal to P/2, the minimum feature size resolvable by such a lithographic system is d=0.5□/NA, where sin(□1)=NA, and thus k1=0.5 as discussed above.
FIG. 1 schematically illustrates an optical projection lithographic system 10 in which illumination light (actinic energy) is provided through the aperture of pupil 12, and collected by a condenser lens 14. An illumination beam 16 is directed to a mask or reticle 18. The light is diffracted by the mask 18, creating diffracted orders m=0, ±1, ±2, . . . , which are then collected by a projection lens 20, and projected to the wafer 22. In the case of off-axis illumination (OAI), the zeroth order beam will propagate undiffracted at an angle □0 from the optical axis 24, but only one of the +1 or −1 diffracted orders, propagating at angle □1, need be collected in order to form an image on the wafer 22. Thus, OAI provides resolution enhancement because the angle collected by the lithographic system will allow a correspondingly smaller grating pitch P to be used. The angle of propagation can be optimized for a primary or target pitch. In addition, if the angle of off-axis illumination is chosen so that zero order and one of the first orders are at the same distance from the center of the pupil of the projection lens 20, the relative phase difference between the zeroth order and that first order will be zero, making the image less subject to defocus, and thus increasing the depth of focus (DOF) for an associated pitch.
The drawback of OAI is that pitches other than the primary pitch will print with degraded process windows. In addition, since there are no discrete diffracted orders for isolated lines, there is little improvement of resolution for isolated lines as compared to densely pitched lines and spaces. The use of sub-resolution assist features provides a means of recovering the process window for pitches that are not enhanced by OAI. By creating nonprinting (non-resolved or sub-resolution) supplementary patterns next to the primary patterns on the mask in such a way that the combined layout approximately reproduces the primary pitch, thus producing the required interference effects, the overall process window can be improved.
Sub-resolution assist features (SRAFs), also known as scattering bars or intensity leveling bars, that are incorporated in photomask layouts, can provide significant lithographic benefit (e.g., improved process window) in the imaging of very large scale integrated (VLSI) circuit patterns when used in conjunction with OAI (e.g., annular illumination). The reason for this is because the SRAFs are designed to optically mimic a dense pattern, but print an isolated one. Methods for selecting size and placement of SRAFs have been discussed in the prior art.
The rules for laying out SRAFs, however, are determined only for grating structures; that is, the sizes and relative placements of the SRAFs are applicable only to geometries consisting of simple lines and spaces (i.e., 1D geometry). Real layouts, however, are more complicated and consist of features that may have finite length, corners, or T-junctions (i.e., 2D geometry). As it is very difficult to enumerate all the possible combination of geometries that may occur in a real layout, SRAF placement rules are not currently derived specially for any of these cases. Instead, the simple rules corresponding to the 1D geometry are applied, and termination of the SRAFs are determined by intersections or manufacturing constraints; such constraints typically dictate the maximum distance between features to be placed on a photomask, the minimum length of features, etc. It is expected that many SRAFs will not meet these constraints; those that do not are typically discarded. Unfortunately, the “cleanup” of an SRAF design containing un-manufacturable SRAF solutions may negatively impact lithographic performance (e.g., reduce process window, reduce yield, etc.).
A layout 30 of features to be printed, including an example of a “raw” SRAF layout prior to cleanup, is illustrated in FIG. 2. In the layout 30, the solid features 32 represent the main features to be printed on a wafer, and the cross-hatched features 34 represent SRAFs. The layout 30 was generated strictly according to a rules table (prior art) with no cleanup. That is, the single “best” SRAF solution that optimized some figure of merit (e.g., process window area) was used to place the SRAFs.
An illustrative 1D SRAF rules table 36 is illustrated in FIG. 3. The SRAF rules table 36 includes columns 38, 40, 42, 44, and 46, labeled as “#A,” “Pitch,” “Line Bias,” “Assist Width,” and “Loc1,” respectively. Here, “#A” refers to the number of assist features used per space (i.e., the distance between two adjacent main features), “pitch” refers to the distance between the center of neighboring main features, “line bias” refers to the width of a main feature on the mask, “assist width” refers to the width of an assist feature on the mask, and “Loc1” refers to the placement of the center of an assist feature relative to the center of a main feature. It is assumed for the purposes of this description that the reader has an understanding of SRAF rules table generation commensurate with one skilled in the art. Accordingly, a detailed description of how SRAF rules tables are provided is not presented herein. A description may be found, for example, in Patent Application Publication No. US 2002/0091985 A1 to Liebmann et al., published Jul. 11, 2002, entitled “Method to Determine Optical Proximity Correction and Assist Feature Rules which Account for Variations in Mask Dimensions,” incorporated herein by reference.
FIG. 4 illustrates the layout 30 of FIG. 2 after cleanup of SRAF features that violate manufacturing criteria (e.g., minimum space between SRAFs and features, minimum SRAF length, etc.). Comparing the regions 48 and 50 in FIGS. 2 and 4, it can be seen that the SRAFs 34 on the left-hand side of FIG. 2 that have touching corners have been shortened to meet minimum spacing constraints, while the SRAFs 34 immediately above the three vertically-oriented features 32 on the right-hand side of FIG. 2 have been removed to meet minimum spacing constraints and minimum feature lengths. Note that, after deleting the violating SRAFs 34, a significant amount of SRAF coverage has been sacrificed. Those areas not covered may suffer a decreased process window and can potentially reduce yield.
The effect of discarding the SRAFs that violate the manufacturing constraints is clear: incorrect line bias and an increase in sensitivity to focus error. The former problem is corrected by a simple re-biasing of the line in order to produce the correct printed linewidth on the wafer for the given nominal exposure dose; this is accomplished by applying Model-Based OPC (MBOPC) and is well known in the art. The latter problem, however, is not easily correctable. Even though MBOPC will correct the linewidth near a missing SRAF at nominal exposure dose and in focus, the local behavior of the linewidth out of focus will change. Because such out-of-focus conditions can be reached simply through imaging on a non-flat surface (i.e., over topography defined by lower layers), the linewidth change through focus is an important consideration.